An electrically-heated catalyst which is energized and heated for effectively cleaning exhaust gas is known from JP05-115795 A, for example. The electrically-heated catalyst includes a carrier and electrodes attached to the carrier for energizing the carrier. The electrodes supply the carrier with electric power supplied from an external electronic power supply such as a battery. The carrier is activated by being energized via the electrodes to be heated. In this way, with the electrically-heated catalyst, by energizing the carrier to heat it, it is possible to effectively clean the exhaust gas.
According to the electrically-heated catalyst which disclosed in JP05-115795 A, the carrier and the electrodes are made of ceramic and metal, and are secured to each other by soldering or welding. However, when the materials with different coefficients of thermal expansion are bonded (i.e., a ceramic-based material with a small coefficient of thermal expansion and a metal-based material with a great coefficient of thermal expansion are bonded directly), thermal stress is induced in the bonded surface due to the difference in the coefficients of thermal expansion. Hence, if the electrically-heated catalyst has the configuration in which the carrier and the electrodes whose coefficients of thermal expansion are significantly different from each other are bonded directly, there is a problem that the electrodes tend to peel off from the carrier due to the thermal stress induced in the bonded surface between the electrodes and the carrier, when they are used continuously under a situation where a temperature change occurs.
It may be contemplated that, instead of direct bonding between the electrodes and the carrier as described above, an underlying layer for releasing the thermal stress is provided between the electrodes and the carrier, and the electrodes as a whole are bonded with a fixing layer on the underlying layer. However, in the configuration in which the electrodes as a whole are bonded to the fixing layer formed by thermal spraying on the underlying layer, the electrodes cannot thermally expand or shrink freely when the thermal stress is induced in the bonding surface between the electrodes and the fixing layer or the bonding surface between the underlying layer and the fixing layer, and thus cracks or ruptures are easily generated at the electrodes, etc.